ABSTRACT Immune system abnormalities have been repeatedly observed in several psychiatric disorders, including severe depression, bipolar disorder and schizophrenia. However, it is not clear whether these alterations are an underlying cause or occur as a result of these disorders. The present study will investigate the role that inflammatory markers, as modulated by genetic make-up, play on regulation of functional and structural brain systems and behavior across the adolescent developmental period, prior to onset of psychiatric disorders. We will perform a longitudinal study of 160 high-risk adolescents, between the ages of 12 and 15 years at baseline, who have a parent diagnosed with depression, but who do not yet themselves have a psychiatric diagnosis. These children, will be compared to a cohort of age matched children (n=160) who have no family history of psychiatric disorders. We will examine blood levels of inflammatory markers in the adolescents throughout five years, and will correlate these levels with genetic variations within genes involved in the immune system, by using high-density genotyping techniques, to assess the role that genetic make-up may play in immune signaling. In parallel, each subject will undergo a series of behavioral and physiologic evaluations to assess several potential risk markers for psychiatric disorders. These evaluations include: a) stressful life events; b) clinical assessment of psychiatric symptoms and disorders; c) dimensional measures of behavior/personality traits including mood, anxiety, impulsivity and hostility; and d) functional and structural neuroimaging. At the time of the grant submission, baseline neuroimaging, DNA, plasma, lymphocytes, and immortalized cell lines have been collected and established in the cohort of 320 adolescents. During the proposed project period, adolescents will be reassessed yearly with the neuroimaging protocol being repeated twice at two year intervals. By performing these longitudinal studies, we will be able to identify how changes in immune signaling, as modulated by genetic make-up and in combination with exposure to adverse environmental experiences during a critical developmental period, may lead to changes in brain development and behavior which may in turn lead to disease onset. These studies will lead to a better understanding of the developmental origins of psychiatric disorders in which inflammation may play an important role.